The invention relates to a circuit for compensating errors in the form of noise and/or distortion from the output stage of a digital amplifier, the digital amplifier comprising: a pulse modulator being adapted to generate a set of control pulses in response to the digital amplifier input signal and a digital compensation signal, a first switching output stage connected to a power supply circuit, the first switching output stage being adapted to receive the set of control pulses and produce the digital amplifier converter output.
Further the invention relates to a method for compensating errors in the form of noise and/or distortion from the output stage of a digital amplifier in which a pulse generator generates a set of control pulses in response to an input signal transferred to the amplifier and generates a digital compensation signal, said output stage comprises a first switching output stage for producing the output from the amplifier.
Conventional digital pulse-width modulated amplifiers usually have very precise power supplies in order to achieve a good sound quality, since any ripple or noise in the power supply voltage is undesired and is transferred to the output and thereby to the loudspeaker. Power supplies for this type of amplifiers therefore typically have a rectifier, a low-pass filter and a regulator. To achieve a desired sound quality, it is necessary to use a complicated and thereby expensive regulator and/or insert large and thereby expensive capacitors and inductors in the power supply filter.
As mentioned above, in connection with digital amplifiers of the above-mentioned type it is well-known that noise from the power supply of the digital amplifier modulates the audio signal. It is difficult to compensate this noise by negative feedback, as is known in connection with analogue amplifiers. This means that if the noise is not compensated in a manner other than by negative feedback, then intermodulation noise/distortion of 5-10% may occur, which is unacceptable.
A known manner, cf. e.g. U.S. Pat. No. 5,559,467, of compensating noise and errors in a digital amplifier comprises feeding the signal of the power supply in a feedforward coupling to an A/D converter which, via a divider circuit dividing an interpolated input signal by the output signal of the A/D converter, is fed to a noise shaper whose output is fed to a pulse width modulator which provides a correction signal that, from the input signal, removes ripple voltages from the power supply. This known way of eliminating ripple voltages, however, requires relatively expensive non-linear circuit components and/or signal computations. For example, is the division process a time consuming task on a digital signal processor.
For the above-mentioned method, it is essential to keep the delay through the correction circuits and digital amplifier to a minimum. Otherwise, the effect of the compensation is strongly reduced, since the compensation action happens too late after the occurrence of the error of the switching output stage.
An improved circuit disclosed by the inventor of U.S. Pat. No. 5,559,467 in the publication xe2x80x9cIEEE, Power Electronocs Specialist Conf. REC., 1995, p 96-102, vol. 1xe2x80x9d, comprises further a predictor filter to compensate for the delays in the feedforward compensation loop (e.g. delays in the A/D converter and in the digital amplifier). The drawback is that the prediction filter is relatively expensive expressed in hardware or commutation cycles of a digital signal processor. Secondly, a prediction filter will normally amplify high frequency noise in the system possibly resulting an adverse effect on the system performance.
Accordingly, an object of the invention is to provide a circuit which is partly more accurate and is partly less expensive to implement than the mentioned known circuits.
This and other objects of the invention is achieved in accordance with one aspect of the invention in that the compensation circuit comprises:
a multiplying D/A converter (MDAC) with an analog input signal and a digital input signal, producing and analog output signal being substantially proportional to the analog and digital input signals,
the analog input of the MDAC receiving an error signal characteristic of errors and or noise in the output stage,
a feedback path from the analog output of the MDAC to the digital input of the MDAC,
the feedback path having a digital section and an analog section,
the digital compensation signal being derived from one or more signals in the digital section of the feedback path.
With a view to a particularly simple design of the regulation loop, it is noted that in one aspect of the invention the feedback path around the MDAC comprises an analog to digital converting circuit separating a digital section of the feedback path from an analog section of the feedback path.
According to another aspect of the invention, the feedback path comprises an analog to digital converting circuit (ADC) adapted to receive an analog signal from the analog section of the feedback path and producing a digital converter output signal feeding into the digital section of the feedback path.
Thus according to this aspect of the invention, the compensation circuit is formed by a regulation loop formed by a feedback path around a multiplying D/A converter with an analogue input which receives an error signal, such as a multiplicative error signal m(t) (to be explained below), characteristic of the output stage on its analogue input, said multiplying D/A converter multiplying the multiplicative error signal m(t) by a compensating signal c(k) which is provided in the regulation loop. The compensation signal c(k) is fed to a second input of the pulse modulator that produces an output signal in response to the amplifier input signal and the compensation signal in order to compensate for the errors in the output stage. This can e.g. be achieved by comprising a multiplication of the amplifier input signal by the compensation signal.
By means of a regulation loop it is now possible according to this aspect of the invention to implement the invention with relatively inexpensive circuit components, as these may consist of simple linear components, as is known in connection with ordinary regulation loops, and without any requirement of huge data calculations, as is required by e.g. a predictor or divider circuit.
According to a further aspect of the invention, the feedback path has a transfer characteristic with a high gain in particular at low frequencies, it is ensured that the errors are compensated in particular well at low frequencies (e.g. in the audio band).
Further, the low-pass filter transfer characteristic of the feedback path, according to this aspect of the invention, will compensate the delays that occur in the compensation circuit. Finally, the low-pass filter characteristic results in a noise shaper effect, which gives the additional advantage that it is sufficient to work with an ADC with lower resolution and precision (e.g. with a much lower number of bits than by linearly coded audio PCM, i.e. typically in the range of 1-10 bits). Thereby the ADC can be realized as a simple and inexpensive circuit that has a low response delay.
In accordance with a still further aspect of the invention, the analog section of the feedback path is adapted to receive a reference input voltage then this reference input voltage can designate the reference point for the compensation circuit, the reference operating state where no compensation is needed.
In accordance with yet another aspect of the invention, the analog to digital converting circuit comprises one or more local feedback path(s), such local feedback paths can help to improve the precision of the ADC circuit or, conversely, to relax the needed precision of the components in the ADC circuit. Further, the local feedback paths can improve the overall stability of the regulation loop.
In accordance with a still further aspect of the invention, the analog to digital conversion circuit produces a one-bit (i.e. 1-bit) digital converter output signal, it is achieved that the ADC circuit as well as other parts of the compensation circuit may be realized by quite simple and inexpensive circuit components with reduced accuracy and tolerance requirements.
As mentioned the invention also relates to a method.
In accordance with another aspect of the invention, the method is characterized in an analog an a digital signal is transferred to the input of a D/A converter for delivering at its output an analog signal proportional to the analog and digital input signals, said output signal being transferred in a feedback path to the digital input of the D/A converter, said feedback path having a digital and an analog section, said digital section being used for deriving compensation signals.